1. Field of Invention
The invention relates to a device for wavelength division multiplexed systems and systems incorporating the device, and more particularly to dynamically reconfigurable add/drop multiplexers with low coherent cross-talk and optical communication networks incorporating add/drop multiplexers.
2. Discussion of Related Art
Demand for optical communication systems is growing with the growing demand for faster broadband and more reliable networks. Wavelength division multiplexing (WDM) is one technique used to increase the capacity of optical communication systems. Such optical communication systems include, but are not limited to, telecommunication systems, cable television systems (CATV), and local area networks (LANs). An introduction to the field of Optical Communications can be found in “Optical Communication Systems” by Gowar, ed. Prentice Hall, NY, 1993.
WDM optical communication systems carry multiple optical signal channels, each channel being assigned a different wavelength. Optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, and transmitted over a single waveguide such as an optical fiber. The optical signal is subsequently demultiplexed such that each channel corresponding to a band of wavelengths is individually routed to a designated receiver.
Single or multiple optical channels can be routed to different destinations, such as in telecommunication networks, cable television subscriber systems and optical LANs. Routing is performed by selectively sending specific channels to a desired location. Another signal may be subsequently added to the dropped or other unused channel. This form of optical routing is generally referred to as “add/drop multiplexing or ADM”.
Fixed wavelength add/drop multiplexers (WADM) are already available commercially. However, such systems require that the wavelengths to be dropped at a specific site, commonly known as a node, be known in advance. Fixed notch filters—typically made from Bragg gratings—are utilized to make fixed wavelength add/drop multiplexers. However, advanced optical networks require that a node be established within the network for any one, all, or any specific set of wavelengths to be dropped, or re-routed on demand. There is thus a strong need for programmable and/or reconfigurable all-optical wavelength add/drop multiplexers (WADM) in such networks.
In order to obtain reconfigurable add/drop multiplexers, optical components capable of directing or routing optical wavelengths are required. Bragg gratings, electromechanical switches, micro-electromechanical systems (MEMS), and liquid crystals are some of the optical components which have been proposed as tuning elements in a reconfigurable add/drop networking element.
Optical add/drop multiplexers based on tunable Fiber Bragg Gratings (FBGs) have been proposed and patented. For instance, in U.S. Pat. No. 6,185,023, Mizrahi describes add/drop multiplexers which are compatible with dense wavelength division multiplexing (DWDM) systems. Mizrahi attempts to solve the problem of cross-talk between dropped and added channels by separating sets of Bragg gratings with an optical isolator. The Bragg grating sets and the optical isolator are interposed between first and second couplers. The optical channels to be dropped from the DWDM optical signal are reflected by the first set of Bragg gratings and exit the add/drop multiplexer through the first coupler. Similarly, in U.S. Pat. No. 6,069,719 and U.S. Pat. No. 5,748,349 of Mizrahi is disclosed a grating-based add/drop multiplexer wherein a set of Bragg gratings is positioned in the transmission path for reflected signals to be dropped.
Sridhar in U.S. Pat. No. 5,778,118 describes an optical add-drop multiplexer for wavelength division multiplexed optical communication systems. The add-drop multiplexer includes an optical filter for selecting portions of a wavelength division multiplexed optical signal. The portions of the wavelength division multiplexed signal which are not sent to an input port exit the add-drop multiplexer.
Giles et al in U.S. Pat. No. 5,754,321 describe an alternative add/drop optical circuit based on fiber Bragg gratings and polarizing beamsplitters. According to that reference, the input beamsplitter means splits the input signal into two different polarized input signals. Each polarized input signal is connected to a first end of a different selective wavelength filter, each of which is arranged to reflect the drop signal back to the input beamsplitter and pass the remaining signal portion to the output beamsplitter.
Liu et al in U.S. Pat. No. 5,953,141 describe an optical add-drop multiplexer and network which can dynamically route on a per-wavelength basis with minimized spectral filtering of the pass-through wavelengths which allows a wavelength to pass through a large number of routing nodes without distortion of the information. Similarly, in U.S. Pat. No. 6,208,443 B1 Liu et al discuss a method and apparatus for constructing an optical wavelength-routing network in which each network node is a dynamic optical add-drop multiplexer (OADM) with minimized spectral filtering effect on pass-through channels and with survivability upon failure.
Huber in U.S. Pat. No. 5,467,212 describes an addressable grating modulation system for an optical cable television system. A tunable optical filter is provided in order to switch video signals onto an optical fiber going to the node in a particular neighborhood. An arrangement uses in-fiber Bragg gratings in order to remove and insert different optical frequencies. The Bragg grating reflects one or more wavelengths and allows passage of wavelengths other than the desired wavelength. Therefore, the desired wavelength is dropped for processing further with other systems.
In the prior art the add/drop multiplexers are mostly based on fiber Bragg gratings. However, an issue of some significance with fiber-grating based tunable add-drop multiplexers is that of coherent cross talk. If a grating with insufficient reflectivity is used in an add/drop multiplexer, an unacceptable portion of the incident channel to be dropped will pass through, resulting in coherent cross-talk with the channel of the same wavelength which is subsequently added within the add-drop multiplexer. To limit this type of cross-talk it is desirable for attenuation of a dropped optical channel to be greater than 30 dB (typically 35 to 40 dB is desirable). While such high reflectivity gratings have been fabricated, the yields for such high reflectivity devices is low, making them very expensive. In addition, very high grating reflectivity is also associated with broader grating bandwidth, which makes these devices unattractive for optical networks utilizing closely spaced optical channels (e.g. 50 GHz spaced DWDM systems).